INVESTIGATIONS OF NORTH AMERICAN CYCADEOIDS: POLLINATION MECHANISMS IN CYCADEOIDEA

Abstract

Wind was at first thought of as the chief pollinating mechanism in Cycadeoidea because of the open angiosperm‐flower‐like reconstruction of the cone presented by G. R. Wieland in 1906. Selfing seemed a more likely possibility after Delevoryas reinterpreted the cone as a closed structure in 1963. However, the existence of what was considered to be a dioecious group of species made it impossible to consider selfing as ubiquitous throughout the cycadeoids. Furthermore, only ovule‐bearing trunks of the species considered to be dioecious were known; no corresponding trunks bearing exclusively microsporangiate cones had ever been found. Recent investigations of a trunk of the type heretofore considered as representative of the dioecious species have revealed that these cones were actually bisporangiate. The discovery that the bisporangiate condition was normal in the cycadeoids eliminates much of the confusion in understanding the system of pollination and allows selfing to be considered the chief means of pollination. Outcrossing probably occurred in very low frequency and could have been the result of a chance wind pollination or of a set of interactions between the cycadeoids and an unidentified animal. Determining the nature of the system of pollination in the cycadeoids makes it possible to speculate on the population structure and evolutionary potential of this important Mesozoic group of plants.